7.1 Elevation

The elevation data for this data set were obtained from the NGDC in Boulder, Colorado, as digitized land elevations (to the nearest meter) for 5´ latitude by 5´ longitude grid cells (i.e., ETOPO 5 data) The NGDC grid contained: < 0 (negative) values for grid cells containing no land within their boundaries; 0 (zero) values for grid cells with land at sea level; and > 0 (positive) values for grid cells with land above sea level. The NGDC grid cells lying along the West Coast were grouped into the 0.25° x 0.25° grid cells used in this data base. Minimum, mean, and maximum elevation data are provided for each 0.25° cell. Each 0.25° cell may contain as many as nine 5´ grid cells. If only one 5´ grid cell within a given 0.25° cell contains a positive or zero data value, then the minimum, mean, and maximum elevation variables will be identical. To calculate and transfer these data from 5´ by 5´ to the 0.25° grid used in this data base, the variables were calculated as follows:

  1. The number of 5´ NGDC grid cells with positive elevation values within each 0.25° grid cell was determined.
  2. The minimum elevation was assigned by finding the minimum elevation of all non-negative 5´ grid cells within each 0.25° grid cell.
  3. The mean elevation was assigned by averaging the elevations of all non-negative 5´ grid cells within each 0.25° cell.
  4. The maximum elevation was assigned by finding the maximum elevation of all 5´ grid cells within each 0.25° grid cell.

To check these data for reasonableness, the 0.25° grid cells were overlaid onto the 1:2,000,000 digitized coastline map of the U.S. West Coast. Through examination of this overlay, it was discovered that peninsulas and small islands often were not represented due to the low resolution of the NGDC grid cells (i.e., mean elevation values were rounded to the nearest whole number). Toovercome this limitation, 0.25° grid cells containing islands and other low-lying landform with negative values in the NGDC data were assumed to lie near mean sea level and were assigned a mean elevation of 0 m. Cells where this correction was necessary are shown in Fig. 3.

A mean elevation value of 0 m indicates that the land within the grid cell has a mean elevation of less than 1 m above sea level. Since 0 is a real data measurement, grid cells with assigned mean elevations are denoted within this data base as having a 0 value for the number of NGDC grid cells variable. A limitation of this method is that even if only a small portion of land is located within the 0.25° grid cell, the entire grid cell contains a mean elevation value of 0 m.

Mean elevation values range from 0 m to 885 m along the West Coast, while the average mean elevation is near 150 m.